1. Field of the Invention
The invention relates to a process for reducing the carbonization of the heat exchange surfaces in a tubular heat exchanger in a unit for thermally cracking hydrocarbons to alkenes, which tubular heat exchanger is used for rapidly cooling the cracking products leaving a cracking furnace.
2. Description of the Prior Art
In the production of, for example, the chemical starting material ethylene, higher hydrocarbons undergo thermal cracking in cracking furnaces. The resulting hot cracking products, which in this case consist largely of ethylene, are cooled after they leave the cracking furnace. For this purpose, heat exchangers are used for indirect heat exchange, and water is used as the coolant to produce usable steam. Devices of this type, constructed as tubular heat exchangers made of boiler steel, are also used in other types of chemical equipment in the cooling and heating of hydrocarbon products.
The effectiveness of a heat exchanger depends heavily on whether deposits which interfere with the transfer of heat are formed on the surfaces of the steel tube (e.g., a tube of 15Mo3) during the operation of the heat exchanger. This is regularly the case in devices in use today. After a certain period of operation, increasing carbonization is found on the side of the heat exchange surfaces which comes into contact with hydrocarbons. It is therefore necessary to repeatedly remove a given heat exchanger from operation after its performance has dropped off by a certain degree, and to subject it to an elaborate cleaning procedure. In modem units, this cleaning is often carried out by passing a mixture of hot steam and air through the heat exchanger on the carbonized side. This loosens and removes the deposits which have formed, so that a metal surface which will ensure good heat transfer from the hot cracking products to the tube walls of the heat exchanger again becomes available.
Even after an extremely thorough cleaning, however, newly formed hydrocarbon deposits appear shortly after the heat exchanger is returned to operation, so that cleaning must be carried out again after a relatively brief period of operation (e.g., after 20 to 60 days). This is undesirable from a technical as well as an economic point of view, because it interferes with steady longer-term operation, reduces efficiency in equipment use, and gives rise to frequent expenditures for the cleaning process itself. For this reason, attempts have been made for years to find solutions which would prevent the rapid carbonization of heat exchanger surfaces.
It has been suggested in EP 0 022 349 Al that a protective film of oxides of the metals Ca, Mg, Al, Ca, Ti, Zr, Hf, Ta, Nb, or Cr be vapor-deposited on the heat exchanger surface that is subject to carbonization. This process is relatively expensive, however, and leads to only limited success.
A similar procedure, i.e., the application of a protective coating, is also suggested in EP 0 110 486 Al. In addition to metal oxides, certain metals, aluminates and silicates are to be used for this purpose. Although this solution also yields improvements, it cannot be considered satisfactory in every case.
Another improvement that is not adequate in every case results from a coating based on silicone resin that is known from NL 84 01 804 Al, in which coating silicon carbide, boron carbide, titanium carbide, silicon nitride or boron nitride is introduced into the resin.
Finally, reference should be made to the attempted use of molybdenum-free steel alloys for the heat exchanger robes. It was once believed that the molybdenum contained in the usual heat exchanger steels had a catalytic effect which promoted carbonization. However, this did not prove to be the case.
The previously suggested solutions have in common the fact that, practically speaking, they are applicable only to new equipment, not to equipment already in operation (except in the case of revamping).